1. Field of the Invention
The present invention relates generally to mulitpath acquisition of dedicated traffic channels in wireless communication systems or networks.
2. Description of the Related Art
FIG. 1 is a frame structure of a dedicated traffic channel for UMTS uplink. Systems or networks designed based on third generation wireless standards such as 3GPP (UMTS) and 3GPP2 (cdma2000) use a dedicated traffic channel in the uplink for communication from mobile users (or user equipment (UE)) to the base station (or Node-B). As shown in FIG. 1, the dedicated uplink traffic channel may include two parts, a data part (Dedicated Physical Data CHannel (DPDCH) in UMTS, known as a Fundamental CHannel/Supplemental CHannel (FCH/SCH) in cdma2000), and a control part (Dedicated Physical Control CHannel (DPCCH) in UMTS, known as a pilot/power control sub-channel in cdma2000).
For the uplink DPCCH in UMTS, there are 15 slots per radio frame (i.e., processing duration corresponding to 15 slots, where the length of the frame is 38,400 chips). One radio frame is 10 ms in duration; thus each slot is 0.667 ms in duration.
The uplink DPCCH may be used to carry control information generated at Layer 1 (the physical layer). Layer 1 control information includes known pilot bits to support channel estimation for coherent detection, transmit power-control (TPC) commands, feedback information (FBI), and an optional transport-format combination indicator (TFCI). The TFCI informs the receiver about the instantaneous transport format combination of the transport channels mapped to the simultaneously transmitted uplink DPDCH radio frame.
Within each slot, the UE thus transmits pilot bits and certain control bits such as TFCI, FBI and TPC bits. Each slot has a total of ten (10) combined pilot bits and control bits. The actual combinations of bit numbers may change and may be controlled by the Radio Network Controller (RNC) at the network, for example. An example configuration may be 5 pilot bits, 2 TFCI bits, 1 FBI bits and 2 TPC bits for one slot.
The pilot bits are known to both the Node-B and the UE; the remaining control bits (TPC, FBI and TFCI) are not known to the base station (Node-B). The number of TPC bits per slot is typically either 1 or 2 bits. If there are two TPC bits in one slot, the values of the 2 bits are identical, i.e., either both TPC bits are 0 or both bits are 1. For 3GPP2 (cdma2000), the frame structure is similar to FIG. 1, although there are no TFCI and FBI bits defined in 3GPP2. For the following discussion, a conventional UMTS transmitter/receiver interface is described.
FIG. 2 is a block diagram of a conventional UMTS uplink transmitter/receiver relationship. Referring to FIG. 2, at the transmitter 200 (of the UE), the Dedicated Traffic Channel/at coding and rate matching block 202. The Dedicated Control Channel (DTCH/DCCH) are coded and merged into one bit stream. This becomes the DPDCH. The DPCCH and the DPDCH are then modulated using BPSK (Binary Phase Shift Keying) at BPSK modulators 205. The DPCCH and the DPDCH are then spread by two different and orthogonal codes (Walsh codes) at 210 and weighted by corresponding gains at 215 to achieve certain power levels. The two channels are then combined (code-division multiplexed) at multiplexer 220. The combined signal may be scrambled and filtered by a shaping filter 225 before modulated to RF (not shown for purposes of clarity) and sent through the propagation channel 230 to the base station (Node-B) receiver 250.
At the Node-B receiver 250, the received signal first passes a matched filter 255. The filtered signal may then be sent to a DPCCH and DPDCH processing block 260 to generate DPDCH soft symbols and a TFCI word for further processing by blocks such as turbo/convolutional decoders (shown in FIG. 2 as a DTCH/DCCH Decoder) to recover the transmitted DTCH/DCCH data. The DPCCH and DPDCH processing block 260 also generates propagation channel measurements such as mobility of the UE. In FIG. 2, for example, this may be shown as a ‘binary mobility indicator’ 264, which may have a value of ‘1’ to indicate a high mobility user and a value of ‘0’ to indicate a low mobility user. This information may be used to improve the multipath acquisition performance for UEs with different mobility.
The DPCCH and DPDCH processing block 260 thus requires the knowledge of propagation paths, primarily the path positions. This knowledge is produced in the receiver 250 by a multipath acquisition block 265 and is managed by an ‘existing and new paths management’ block 270. The multipath acquisition block 265 searches a possible range of path positions (also occasionally referred to herein as ‘paths’ or ‘hypotheses’) and reports all positions that are determined as having significant signal energy, such as above some given threshold.
The existing and new paths management block 270 further screens the paths reported by the multipath acquisition block 265 and the paths that are already in use in the DPDCH and DPCCH processing block 260. The existing and new paths management block 270 removes repetitive paths and/or weak paths, adds new paths just discovered by the multipath acquisition block 265 and then passes the updated paths' information back to the DPDCH and DPCCH processing block 260. The frequency of the update can be programmable, depending on the design goals. For example, an update interval or frequency may be one DPCCH frame (10 ms). As will be seen below, conventional multipath acquisition uses only the pilot signal information in the DPCCH.
FIGS. 3A and 3B illustrate process flows for multipath acquisition of a dedicated traffic channel. In particular, FIGS. 3A and 3B generally describe the processing in multipath acquisition block 265 of FIG. 2. This processing flow is described in U.S. patent application Ser. No. 11/090,064 by the inventors and entitled “METHODS OF MULTIPATH ACQUISITION FOR DEDICATED TRAFFIC CHANNELS”, filed Mar. 28, 2005, hereafter the “'064 application”.
Present and future 3GPP/3GPP2 wireless communication systems should be able to support high mobility users. One example application is a user making phone calls from a high-speed train. The record velocity on a commercial high-speed train is the MEGLEV in Shanghai, China, which travels at speeds in excess of 480 Km per hour. For this velocity, the maximum frequency shift for a UMTS system operating in the 2 GHz band is around 2 KHz.
Taking half a cycle of a sinusoid as the coherence interval, then with this frequency shift there is a coherence interval of about 0.25 ms. This presents a substantial challenge to a single pilot signal processing block used in the prior art because the pilot accumulation interval in one slot may exceed the coherence interval of the channel by a substantially large margin. For example, in UMTS, there may be a maximum of 8 pilot symbols per slot, out of the 10 total symbols in that slot. The pilot interval in this case is 0.5333 ms, larger than the 0.25 ms cycle of the frequency shift. In this case, if the pilot signals are still accumulated in the slot, the signal energy is more or less cancelled to zero (considering the accumulation sinusoidal in one cycle, the output is zero). Therefore, a modification is made to provide additional pilot processing blocks so as to more efficiently handle high mobility UEs.
Referring to FIG. 3A, and as described in the '064 application, the pilot energy over a frame is calculated for a specific path position (hypothesis). Initially, the matched filter output from matched filter 255 corresponding to this hypothesis (which is a complex signal) is descrambled and despreaded (310). The pilot pattern is also removed by function 310 as well.
To handle frequency shifts as high as 2 KHz, the pilot signal is bi-sected or divided into two segments (320a, 320b). One segment consists of four (4) pilot symbols, which would have an interval of 0.26667 ms and would barely satisfy the coherence interval for 2 KHz, but nonetheless maximizes the coherence accumulation gain. The other segment has 4 or fewer pilot symbols, with an interval less than or equal to 4 symbols (≦0.26667 ms) since in 3GPP the largest number of pilot symbols in a slot is 8. The pilot symbols within each segment are accumulated (320a, 320b) before the calculation of their corresponding L2-norms (330a, 330b).
Next, the L2-norms of the outputs from 320a and 320b are formed (330a and 330b). Assuming for example that the complex output signal is z=a+j*b, its L2-norm is given by L2(z)=a2+b2. The L2-norms of the accumulated pilot signal are further accumulated over a frame interval (340).
As shown in FIG. 3A, the binary mobility indicator 264 from FIG. 2 is used to decide whether a bisect segmentation is needed or not. This preserves the acquisition performance for lower or low mobility users, as lower or low mobility users would not require segmentation on the pilot signals in a slot, and therefore would retain higher coherent accumulation gain. Thus, binary mobility indicator information is used to improve multipath acquisition performance for high mobility users while retain the high performance gain from coherent pilot combining for low mobility users.
In addition to the pilot signal processing at 320a/b and 330a/b, three additional processing block groups are included for processing control information such as output symbols corresponding to TFCI bits (322 and 332), FBI bits (342 and 334) and TPC bits (at 326 and 336).
As TFCI and FBI control bits in a given slot are unknown to the Node-B, output symbols corresponding to these control bits cannot be accumulated (see 332 and 334) prior to the L2-norm calculations at 322 and 332. Otherwise, the signals may cancel one another due to opposite-polarity signs of the symbols. On the other hand, the Node-B knows that if there are multiple TPC bits in one slot, they have to be identical.
Therefore, the output symbols corresponding to the TPC bit(s) in a slot are accumulated (326) prior to being subject to the L2-norm calculation (336), in the same way as the pilot symbols are processed. The L2-norms of the accumulated TFCI, FBI and TPC symbols are further accumulated over the frame interval (340). Since additional energy is collected, the probability that new paths are discovered and existing paths are maintained is increased. Equivalently, to maintain the same probability of detection and or maintain the paths, the UE now can transmit as a lower power level, therefore reducing interference to other users in the cell. The resultant output is the DPCCH frame energy (350).
Referring to FIG. 3B, the DPCCH frame energy for each hypothesis (355) is compared with a fixed pre-defined or given threshold (365). Hypotheses with DPCCH frame energy surpassing the threshold (output of 365 is ‘YES’) are reported (375) to the existing and new paths management block 270 in FIG. 2 for further processing.
The multipath acquisition process flow described in the '064 application, while configured to more efficiently handle high mobility UEs, uses only the DPCCH signal energy to detect the propagation paths.